1. Field of the Invention
This invention relates to a novel synthetic lubricating oil and more particularly to a novel synthetic lubricating oil which is excellent particularly in oxidation stability and comprises, as the main component, at least one mono-substituted naphthalene derivative having a specific structure.
2. Prior Art
Lubricating oils are generally required to have a long-term service life. To meet this requirement, there has usually been used a lubricating oil prepared by adding, as required, a suitable antioxidant to a highly refined mineral oil. It is difficult, however, to use a mineral oil as a lubricant for a long period of time under severe temperature conditions since the mineral oil has limited oxidation stability. Thus, as lubricating oils having better oxidation stability, there have been developed and widely used ester-type synthetic oils such as diesters and polyol esters, and hydrocarbon-type synthetic oils such as poly-.alpha.-olefins and alkylbenzenes.
However, although these known synthetic lubricating oils are appreciated to have higher oxidation stability than mineral oils, they are still not satisfactory in stability to oxidation.
The present inventors made intensive studies in attempts to develop synthetic lubricating oils having further higher oxidation stability and, as the result of their studies, found that lubricating oils comprising, as the main component, at least one monosubstituted naphthalene derivative having a specific structure, have remarkably high oxidation stability as compared with the conventional known synthetic lubricating oils. This invention is based on this finding or discovery.